Droplet microemulsion scattering from

Scattering from droplet microemulsions 2.2.1 General outline... 

In this subsection, the theoretical background for SANS and neutron spin-echo measurements carried out with o/w- and w/o-droplet microemulsions will be presented. According to Milner, Safran and others, shape fluctuations in droplet microemulsions can be described in terms of spherical harmonics [42-44]. This offers the possibility to calculate a dynamic structure factor S(q,w) or its Fourier transform, i.e. the intermediate scattering function I(q,t) for the problem, which can be used to analyse dynamical measurements by neutron spin-echo spectroscopy [45]. For the scattering from thin shells I(q,t) was calculated [43]... 

As already pointed out the first work directly measuring the deformation dynamics in an o/w-droplet microemulsion using NSE was published by Huang et al. [45]. In this work, a microemulsion based on the surfactant AOT was studied and it was shown that the intermediate scattering functions contain information about the centre of mass diffusion and in addition also contributions from the deformation dynamics. The intermediate scattering functions obtained in this work are shown in Fig. 2.3. 

Figure 2.3 Intermediate scattering functions obtained for an AOT-based o/w-droplet microemulsion using NSE. The solid lines are fits with a single exponential function yielding an effective diffusion coefficient. Note that this was the first NSE study of a microemulsion showing the calculation of k on the basis of the intermediate scattering functions. (From Ref. [45], reprinted with permission of the American Physical Society.)...

Figure 2.5 Relaxation rates F of the intensity correlation functions as a function of q2 obtained via a photon correlation spectroscopy experiment. The sample was a w/o-droplet microemulsion made of D2 0/n-octane-di8/CioE4. On the oil-continuous side of the phase diagram the scattered light intensity is usually low leading to rather large errors of the individual data points. Nevertheless, from the slope of the linear fit the translational diffusion coefficient is obtained. (Figure redrawn with data from Ref. [67].)...

Figure 2.6 Measured intermediate scattering functions of a w/o-droplet microemulsion for the system D2 0/n-octane-d- 8/CioE4. The four curves were obtained at four different q values close to the minimum of the droplet form factor. The solid lines are double exponential fits with only two adjustable parameters. The translational diffusion coefficient was determined using PCS (see Fig. 2.5) and used as input for the analysis of the NSE data. (Figure redrawn with data taken from Ref. [67].)...

The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7Hi5)-(C7Fi5)CHS04 Na [32], 667 g/mol PFPE-C00"NH4 [33], and for a partially fluorinated di-chain sodium sulfo-succinate surfactant [34]. For the PFPE-COO NH4 surfactant, the droplet radius increases from 20 A to 36 A for W o values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 A as Wo increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and Wq were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 

Figure 18.4. Static light scattering from water-in-oil microemulsion droplets with the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT). The normalized static scattering intensity < > is shown as a function of the...

The interpretation of the bulk properties of the microemulsions phases, close to Sx, in terms of critical phenomena, is then less satisfying. Near this boundary, the samples are further from a critical consolute point than in the case of the boundary S2. As far as bulk properties are concerned, light scattering experiments are rather sensitive to droplets elongation as it will be observed in viscosity measurements. 

The analysis of the light scattering data using CONTIN also allows for a determination of the size polydispersity of the microemulsion droplets, because all the moments u = / pm G(r)rndr which describe the distribution function G(T) are computed (for details see Ref. [98]). The polydispersity index is obtained from... 

Figure 7.8 Schematic scattering length density (sld or p x 1010 cm-2) profile fitted to SANS data from surfactant-stabilised D20-in-C02 microemulsion droplets. (From Ref. [55], reprinted with permission of Wiley-VCH Verlag GmbH Co. KgaA.)...

The ease of emulsion formation increases and the droplet size achievable decreases as the interfacial tension falls. Systems in which the interfacial tension falls to near zero j<10-3 mNm (dyne cm-1)] may emulsify spontaneously under the influence of thermal energy and produce droplets so small (<10 nm diameter) that they scatter little light and give rise to clear dispersions. The micro emulsions so formed occupy a place between coarse emulsions and micelles. They are usually effectively monodispersc and unlike coarse emulsions are thermodynamically stable. Microemulsion droplets have sometimes been classified as swollen micelles. In fact, there probably exists an essentially continuous sequence of states from association colloids to coarse emulsions,... 

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